1. Field of the Invention
The invention concerns coherent Doppler radars with variable blind speed, and more particularly to a method and device for compensating for the speed of the clutter.
Pulse Coherent Doppler radars with constant blind speed, by exploiting the Doppler effect, enable moving objects, which give rise to radar signals of low intensity, to be detected amongst fixed objects which give rise to large amplitude radar signals. In effect, in these pulse radars, waves received after reflection from moving obstacles have a phase which varies from one period of repetition to the next. However, waves reflected from fixed obstacles do not have such a variation in phase shift. By virtue of this fact, the signals corresponding to moving obstacles display, after demodulation, components which vary sinusoidally at a frequency fd, generally called the Doppler frequency, which is linked to the radial speed v and to the wave length .lambda. of the radar signal by the formula: ##EQU1## Of course, the signals corresponding to fixed obstacles have a constant amplitude and their spectrum includes a series of discrete lines at the frequencies of F, 2 F, . . . mF; F being the frequency of repetition of the pulses transmitted. Moreover, the spectrum of the pulses corresponding to moving obstacles is composed of discrete lines of the mF.+-.fd type.
One can therefore understand that it is possible to get rid of the signals corresponding to fixed obstacles by using an echo elimination filter which only passes signals at frequencies close to zero. Depending on how wide or how narrow the rejection bandwidth of this filter is, one can also eliminate moving obstacles presenting low Doppler frequencies, for example, clouds, or again, fluctuating fixed obstacles that present a certain Doppler speed, such as trees moving in the wind. These various low velocity spurious echos are known as "clutter".
By virtue of the fact that the rejection bandwidth of the anti-clutter filter is not infinitely narrow, the moving echoes objects which have Doppler frequencies not only equal to mF but close to these latter, cannot be detected. It is therefore said that such a coherent Doppler radar with pulses of constant recurrence frequency presents "blind speeds".
To get over these blind speeds difficulties, one can transmit pulses, for example, which are separated by variable intervals of time. One can thus understand that certain moving echoes which were invisible, because confused with one of the mF lines, should "reappear", whilst fixed echos remain eliminated whatever the variation in these intervals of time between pulses.
In such radars, to allow for the appropriate treatment of the signals received, with the minimum of equipment, it is arranged that the time interval variations between consecutive pulses should follow certain laws or rules. Thus, it is sometimes arranged to transmit the pulses in bursts of q pulses which have, in a given burst, the same carrier frequency and the same recurrence frequency. In other words, from one burst to the next, the carrier frequency and the frequency of recurrence of the pulses may change. In such coherent Doppler radars with pulse burst emission, the blind speeds vary from one burst to the next in such a manner that the moving echos corresponding to the blind speeds of one burst are detected during the course of the other bursts which have different blind speeds.
In such radars, it is important to remove not only the fixed or pseudo-fixed echos, but also any other spurious echoes making up the clutter which have a large radial velocity. To this end, it is necessary to measure the radial velocity of the clutter and to compensate for this so as to bring the spectrum of the interference within the rejection zone of the filter, in order to eliminate them.
2. Description of the Prior Art
The commonly used methods for carrying out this compensation are based on the hypothesis that the clutter zone has a broad spread and has homogeneous characteristics in space and that it is sufficient to measure the average speed of a part of the zone. This measurement of speed is then used to "re-centre" the filter within the whole clutter zone.
This hypothesis of the wide spread and homogenity of the clutter is not always verifiable and low intensity spurious echoes are encountered which are of low spread both in terms of distance and direction and are of radial velocity markedly different from one interference echo to another within the same zone. This is the case of the echoes from eddies forming on the surface of the sea, where the radial velocities are very variable within a relatively small volume.